The present disclosure is related to the general field of antennas design, and particularly pertains to the subjects of electronic beam-steering, feed network synthesis, and electromagnetic metamaterials. The present application describes a beamforming apparatus that may be used for feeding circular ring arrays with 360° scan coverage, its basic design method, and its implementation as an electrical network.
Beam steerable antennas with 360° scan coverage have long been a subject of interest to the antenna community and have been extensively studied for the purpose of implementing radars and direction finders (DF). 360° beam-steering also has many potential applications in wireless communication systems, for example when implementing base station antennas for cellular communication or for wireless local area networks (WLAN).
360° beam steering can be implemented using circular ring antenna arrays, which in this case refers to an array of identical antenna elements arranged on a circle such that each element has an outward directed beam in a radial direction defined by the geometric center of the ring and the phase center of the element. In one implementation, a steerable beam is obtained by switching the electric signal between the different antenna elements, or, in the case of direction finders, by directly reading the received signal at the output of individual elements. In this case, each element corresponds to a predefined radiation pattern and beam.
Alternatively, the beam can be synthesized by combing the radiations (or received signals) from multiple array elements. In this case a large portion of the array elements contribute to the transmitted or received signal at any given time. Distributing the transmitted power between the radiating elements and combining the received signals is achieved by a multi-port electrical network called a “beam-forming network”, or BFN.
BFN is used to realize the desired transfer functions between the antenna elements and transmit/receive electronics. The ports at which the BFN couples to antennas are called “array ports”. The ports at which the BFN is coupled to the electronics are called beam port(s). BFN can be fully passive and made up of fixed components, in which case it will be used to generate a number of pre-determined radiation patterns (multi-beam antenna), or it may be active and include control devices that are used to set the amplitude and phase of excitation for individual antenna elements on command, thereby providing an electronically control radiation pattern and beam angle (phased array). In the contemporary language of antenna engineering, the term beam-forming network usually refers to the first configuration and is used in the context of multi-beam antennas. The same meaning of the term is intended throughout this writing.
There are a number of designs for implementing the BFN for linear antenna arrays (arrays in which the antenna elements are arranged on a straight line), including “Butler matrix” (J. L. Butler and others “Beam Forming Matrix Simplified Design of Electronically Scanned Antennas,” Electronic Design, pp 170-173, 1961) and “Rotman Lens” (W. Rotman and Others, “Wide-Angle Microwave Lens for Line Source Applications,” IEEE Transactions on Antennas and Propagation, pp. 623-632, 1963). In these types of designs, the BFN can be implemented in the form of a rectangular network, where the array ports and beam ports are built on two opposite sides of the rectangle. The phase delay relationships between the array ports and beams port are such that exciting the BFN from any given beam port creates the phase gradient necessary for producing a beam in one of the desired directions. The power entering BFN from each beam port distributes (nearly) equally between all antenna elements.
It is well understood that the most suitable geometry for the BFN in the case of circular ring arrays with 360° scan is a circular geometry. In this case, the beamforming network plays two roles: 1) it produces the desired phase relationship among array ports and 2) it concentrates the power entering from each beam port predominantly among the antenna elements facing the direction of the desired output beam, that is, those whose maximum radiation occurs within ±90° of the output beam angle. We refer to these elements as the “facing elements”. Designing a BFN that accomplishes both of these tasks is difficult.
The design of circular BFN's has been addressed in the U.S. Pat. Nos. 3,392,394, 5,274,389, and 3,754,270. All of these works rely on two-dimensional (2D) Luneberg lenses (G. D. M. Peeler and Others, “A Two-Dimensional Microwave Luneberg Lens,” IRE Trans. Antennas and Propagation, pp. 12-23, 1953) or homogeneous approximation thereof to accomplish the objectives of concentrating the power on the facing array ports and synthesizing the required phase delays. However, due the fact that the radiating aperture and focal point of the Luneberg lens both lie on its outer surface, in all of these designs the ports are defined on the rim of the lens double as both beam ports and array ports. Each of these designs proposes a way for separating the beam ports and array ports, but the resulting assembly is invariably cumbersome and for most applications undesirable.
The beamformer proposed in the U.S. Pat. No. 3,392,394 is implemented as a pair identical 2D Luneberg lenses each having an N number ports that are stacked so that their ports line up on top of each other. Each of the N resulting double port stacks can be converted to a beam port and an array port by introducing a quadrature phase hybrid coupler. The drawback of this design is that it requires two Luneberg lenses and N hybrid couplers that can occupy considerable space and introduce at least some loss into the system.
A simpler version of the above design has been proposed in the U.S. Pat. No. 5,274,389 for a direction finder, where one of the Luneberg lenses is removed and the idle ports of the hybrid couplers is terminated by matched resistors. This approach results in 6 dB loss in the signal that is unacceptable in many applications.
The U.S. Pat. No. 3,754,270 proposes another beamformer that is based on a single N-port lens coupled to N isolators. If the isolators are coupled to the lens ports at their first and are right handed (allowing 1-2-3 rotation), in the transmit mode their second ports can act as the array ports and their third ports as the beam ports. The presence of the isolators adds significant complexity to the design and is particularly problematic at higher frequencies. Also, for the same configuration to work on the receive mode, the sense of rotation in the isolators must be reversed (3-2-1 rotation). This requires the use electronically controllable isolators (such as Farady rotation isolators (D. M. Pozar, Microwave Engineering, 3rd Edition: Wiley, 2004)) that further complicates the implementation.
From these examples it is clear that the lack of separation between the array ports and beam produces undesirable results. Introducing additional building blocks to make this distinction leads to complex configurations that are often large, lossy, not realizable at high frequencies, and not suitable for miniaturization and integration.